The present invention relates to an anodic bonding method and a method of producing an acceleration sensor.
An anodic bonding has been used for bonding, for example, a semiconductor substrate and a glass substrate. The anodic bonding has also been used for bonding silicon substrates with a glass layer in between to produce a silicon structure having a micromachining sensor using Micro Electrical Mechanical System (MEMS) technology. A conventional anodic bonding apparatus has a pair of electrodes having two electrodes plates facing each other, and two substrates to be bonded are placed horizontally between the electrodes. In bonding, one of the electrodes becomes an anode and the other of the electrodes becomes a cathode, so that a voltage is applied to the two substrates. The two substrates closely contact with each other, and portions contacting with each other are bonded through the anodic bonding.
The anodic bonding will be explained next in more detail. When a silicon substrate and a glass substrate are bonded, for example, an electrode on a side of the silicon substrate becomes a cathode and an electrode on a side of the glass substrate becomes an anode. When a voltage is applied to the electrodes, the silicon substrate is charged with positive charges. In the glass substrate, sodium ions are attracted toward the electrode, so that a side of the glass substrate facing the silicon substrate is charged with negative charges. Accordingly, a large static attractive force is generated between bonding surfaces of the silicon substrate and the glass substrate due to the positive charges and the negative charges, thereby bonding the silicon substrate and the glass substrate together.
There is a case that a substrate to be bonded is not completely flat and may have a concave or convex portion. In this case, bonding surfaces of such substrates do not closely contact with each other. Accordingly, an undesirable space may be formed between the bonding surfaces upon bonding, thereby causing an un-bonded portion.
Patent reference 1 has disclosed an anodic bonding apparatus to solve the problem described above. In the anodic bonding apparatus, a silicon substrate is mechanically curved in a convex shape toward a bonding surface thereof, and the silicon substrate is bonded to a glass substrate in this state. FIG. 6 is a view showing the anodic bonding apparatus disclosed in Patent Reference 1. As shown in FIG. 6, a silicon substrate 20 is arranged to face a glass substrate 10, and a slider 30 pushes upwardly an outer periphery of the silicon substrate 20 at four positions. An upper electrode pushes a center portion of the silicon electrode 20 downwardly, so that the silicon substrate 20 is curved in a convex shape toward a bonding surface thereof. Then, the slider 30 gradually moves outwardly, so that the silicon electrode 20 is bonded to the glass substrate 10 from the center portion toward the outer periphery thereof while releasing the curved state.
Patent Reference 2 has disclosed another bonding technology. In the technology disclosed in Patent Reference 2, after a thin layer is formed on a silicon substrate so that the silicon substrate is curved, the silicon substrate is bonded to a glass substrate with the anodic bonding. FIGS. 7(a) to 7(d) are views showing a process of the anodic bonding disclosed in Patent Reference 2.
As shown in FIGS. 7(a) to 7(d), first, a first oxide layer 20 is formed on a bonding surface of a first silicon substrate 10. Then, a second oxide layer 30 having a thickness larger than that of the second oxide substrate 30 is formed on a surface of the first silicon substrate 10 opposite to the bonding surface thereof. Due to the difference in a thickness between the first oxide substrate 10 and the second oxide substrate 30, the first silicon substrate 10 is curved as shown in FIG. 7(a). In this state, the first silicon substrate 10 is bonded to a glass substrate 40. As a result, the glass substrate 40 is curved as shown in FIG. 7(b). Then, the glass substrate 40 in a convex shape toward a bonding surface thereof is bonded to a silicon substrate 50 as shown in FIGS. 7(c) and 7(d).    Patent Reference 1: Japanese Patent Publication No. 07-183181    Patent Reference 2: Japanese Patent Publication No. 09-246127
In the conventional anodic bonding apparatus disclosed in Patent Reference 1, the slider and the upper electrode are provided around the substrates for applying a mechanical stress to the substrates. Accordingly, it is possible that the substrates may be cracked or tipped at the periphery or the center portion thereof. Further, charges move toward the center portion of the substrate, thereby reducing bonding strength at the periphery thereof.
In Patent References 1 and 2, the substrate to be bonded is curved, that is, the substrate curved in a concave shape toward the bonding surface thereof is curved in a convex shape toward the bonding surface thereof by applying an external force. Accordingly, a surface area of the substrate where an element is formed is changed to a relatively large extent, and an excessive stress is generated in the substrate. Especially, when a semiconductor substrate has a small thickness, such a substrate may be damaged due to the excessive stress.
When a substrate includes an element for measuring an external stress such as a silicon structure formed with the MEMS technology, i.e., an acceleration sensor, such a substrate is susceptible to an excessive change in a shape thereof. Accordingly, when such a substrate is bonded, the element itself may be damaged.
In view of the problems described above, an object of the present invention is to provide an anodic bonding apparatus capable of solving the problems described above. Further, an object of the present invention is to provide an anodic bonding method and a method of producing an acceleration sensor.
Further objects and advantages of the invention will be apparent from the following description of the invention.